Tree Fruit Disease Triage

In many ways this is the perfect storm for the tree fruit grower. Even if you were diligent and had good coverage prior to today’s rain, even the best fungicides are removed after two inches of rain. Rainfall totals in Gloucester County already totaled nearly an inch by 7 am Wednesday morning so almost certainly all coverage will have been removed by the time this is over.

Conditions today have been optimal for apple scab, while conditions tomorrow are forecast to be optimal for brown rot blossom blight in stone fruit, and there is a concern that fire blight may be an issue for apple and pear. So growers may be asking themselves, “I’ve only got a few hours to get everything covered, I can’t possibly get across all of it, how do I prioritize?”

Let’s try to break this down.

First consider what your coverage was before this rain. If you covered after Friday’s rain then you had the best situation and were probably covered through Wednesday afternoon. If you were covered before Friday’s rain but not after then you probably lost coverage sometime Tuesday night or Wednesday morning. This is important when considering back action.

Second, consider what your inoculum load is. Basically, was disease controlled last year or was there a control failure? Sanitation measures done over the winter will help but disease pressure will still be higher if the disease was not controlled last year. So if for example you had apple scab last year but brown rot was controlled then your priority is to get the apples covered. If you controlled apple scab but fire blight was a problem, then strep sprays might take priority if weather conditions are favorable.

Third, decide on a control strategy. Let’s look at this by disease:

Apple Scab:

If you had scab last year this should be your number one priority. Even if you were covered just before the rain you will have lost coverage and will need to try to reach back to the point at which coverage was lost. Not the ideal situation for resistance management. We know little about the newer chemistries and premixes so while these may have some desirable properties they cannot be counted on for reach back, especially if resistance to one of the components is already present. The following is the best information we have. If the DMI’s are working for you, Rally and Procure will have 96 hours post-infection activity at the high rates and 72 hours at the lower rates. Indar, also a DMI, likely has no post infection activity, but may have some pre-symptom suppression of lesions. A poor choice in this situation. Inspire Super, a DMI/AP premix is reported to 48 hours post-infection activity. Remember that relying on the DMI’s to control scab after the fact will only speed the development of resistance, and if you suspect the DMI”s are not working avoid them. The QoI’s, Flint and Sovran are reported to have 48-72 hours post-infection activity. The same caveats about DMI resistance go for the QoI’s, perhaps even more so. The AP’s Vangard and Scala are reported to have 48-72 hours post-infection activity. The protectants Captan and EBDC’s are reported to have 18-24 hours post-infection activity. If possible the best approach would be to reapply within 24 hours from the time you think coverage was lost with an EBDC or an EBDC/captan combination. If this is not possible, the next best approach might be to combine 2 different chemistries with similar hours reach back plus a protectant, for example Rally plus Vanguard plus an EBDC , assuming no DMI resistance, so that you aren’t exposing the scab fungus to only one chemistry.

Scab is not the only disease to be concerned about at this stage as powdery mildew and rusts are also active. We have no information about reach back for these diseases.

Brown Rot of Stone Fruit:

Plums and Peaches are near petal fall in southern counties except for some late blooming varieties like Gloria. Cherries are just past full bloom. The priority here should be for cherries and late blooming peach. Once petal fall begins the susceptibility to blossom blight begins to decline. However with more wetting and temperatures forecast in the mid 70’s Thursday conditions are good for infection, and susceptible blossom are still present. Again not the best situation for resistance management. We have little information about brown rot fungicide characteristics but anecdotal evidence suggests that Rovral (Meteor; Iprodione) or Rally or Orbit (Bumper, Propiconazole) should have some post-infection activity assuming no resistance. Avoid Gem where much bloom is present. Rally and Orbit are also best avoided unless no other choice is available. Rovral is probably the best bet if it can be applied before more wetting or shortly after. Use the high rate at 2 pts./acre if coming in after.

Fire Blight:

This could be an issue Thursday but since we are just coming into bloom and it has been relatively cold up until now so it should be third on the list. Still if you have a history of fire blight and have susceptible varieties, be cautious. If you have wetting and it is in the mid to upper 70’s blossom infections could be possible.